Self defense - protecting oneself from physical or emotional harm - is a moral imperative. This need not always entail physical violence however, as in the case where talking can de-escalate a potentially violent confrontation (as in NVC), or for example in martial arts, Judo is the 'way of suppleness' which from what I understand often involves redirecting the opponents own force and energy back against themselves, rather than initiating force or attack of one's own. Dale Brown is an inspiration in this regard.

That said, if someone aggresses against you through no fault of your own, in other words they start it, and you happen to be the one to finish it, hurting them in the process to save yourself, then any harm they suffer is their own responsibility.